1. Field of the Invention
The present invention relates to an optical scanning device using a plurality of light beams, and an image forming apparatus having the optical scanning device.
2. Description of the Related Art
Conventionally, as an image forming apparatus which forms an image by means of the Carlson process, there is known, for example, an image forming apparatus which forms an electrostatic latent image on a surface of a photoconductive drum by scanning the surface of a rotating photoconductive drum with optical beams, visualizes the electrostatic latent image as a toner image using toners, and forms an image by fixing the toner image onto a recording medium such as paper. Recently, this kind of image forming apparatus is often used for simplified printing as an on-demand printing system. There are the further increased demands for high density images and a high speed of image output.
As a method of increasing the speed of the image output, a method of increasing a printing speed by increasing the rotation speed of a polygon mirror, for example, which deflects a light beam, and the rotation speed of the photoconductive drum is generally considered. However, if the rotation speed of the polygon mirror is increased, the noise and vibration from a driving system are increased and the power consumption is also increased, causing a decrease in the durability of the apparatus. Moreover, since the increase in the image output speed has a trade-off relationship relative to the increase in the image density, if the rotation speed of the polygon mirror is increased, the image quality is deteriorated along with the increase in the rotation speed.
Consequently, as a method for simultaneously achieving the high density of an image and the high speed of image output, there is proposed an image forming apparatus which changes a light source into multibeams, and scans the photoconductive drum by a plurality of light beams at one time, as disclosed in JP2005-250319A and JP2004-287292A. In such an image forming apparatus, the light beams from a surface-emitting laser having a plurality of light-emitting parts are together deflected by the polygon mirror, so as to simultaneously scan the surface of the photoconductive drum by a plurality of light beams.
For example, in the surface-emitting laser which is represented by VCSEL (vertical cavity surface emitting laser), and used in this apparatus, the light-emitting diodes are easily two-dimensionally arranged in one element, so the number of light beams used in the single scanning can be easily increased, compared with the end-face-emitting laser. However, it is difficult for this type of the surface-emitting laser to increase the output, and if the arrangement interval between the light-emitting parts is too short, the operating life of the element is significantly deteriorated by the heat interference between the light-emitting parts.
The invention disclosed in JP2001-272615A, for example, is proposed relative to the above problems. In this invention, each of the intervals of the light-emitting parts adjacently formed in the light source is equalized, so as to decrease the influence of the crosstalk by the heat from the light-emitting parts, and to maximize the arrangement density of the light-emitting parts. However, in this invention, the arrangement of the light-emitting parts is complex, and can not be easily achieved. The invention does not disclose a technique which reduces variations of the intervals of the scanning lines.
In addition, the invention disclosed in JP2005-250319A is an invention which combines a light source in which light-emitting parts are two-dimensionally arranged and a scanning optical system which scans the light beams emitted from the light source. However, this invention does not substantially disclose the arrangement of the light-emitting parts or a technique when conducting optical scanning by means of a light source having several tens of light-emitting parts.